Commercially available chemicals have different levels of purities. They may be of commercial grade having a higher level of impurities than reagent grade having negligible amount of impurities. Chemicals labeled “technical” or “commercial” are usually quite impure. An analytical reagent is a chemical almost free from impurities or having minimum level of impurities. Analytical grade sodium chloride is the pure form of sodium chloride having negligible amount of impurities. NaCl is most commonly used in food processing and biological products. This is also one of the most important raw materials of chemical industry [1].
Khewra Salt Mines is a salt mine located in Khewra in Jhelum District of Punjab, Pakistan, about 160 kilometers from Islamabad (capital city of Pakistan) and 260 kilometers from Lahore. Salt has been mined at Khewra since 320 BC, in an underground area of about 110 sq. km. Khewra salt mine has estimated total of 220 million tones of rock salt deposits. The production from the mine is about 325,000 tons salt per annum. Only 50% salt is extracted and 50% is left as pillers to keep the mountain. The salt-mine is 288 meters above sea level and extends around 730 meters inside the mountains from the mine-mouth. The cumulative length of all tunnels is more than 40 km. Salt occurs in a Pre-Cambrian deposit in the form of an irregular dome like structure. There are seven thick salt seams with a cumulative thickness of about 150 meters. Appearance of Khewra salt is transparent, white, pink and reddish to beef-color red [2-4].
Sodium chloride is composed of two elements, sodium and chlorine. The percentage of these elements is Na 39.4%, Cl 60.6% in a unit formula. The rock salt is not always found in pure state. Calcium sulphate and magnesium sulphate are the major impurities present in industrial salt [5-6]. Impurities are mostly mechanical such as droplets of brine, gas bubbles, and also inclusions of clay and organic matter, gypsum, KCl, MgCl2, CaCl2, Na2SO4, MgBr2, MgI2, and MgSO4. Salt is the most widely distributed mineral and has four distinct modes of occurrences: (1) Extensive deposits of rock salt; (2) salt solutions; (3) as sublimation products near volcanoes; and (4) as efflorescent, earthy crusts in arid regions. Out of these types only the first two are of commercial importance [7]. Synowiec [8] utilized the waste brine from an evaporative salt plant by means of spent solution from ammonia soda production. Rock salt is used for the production of purified salt by ordinary mining. Evaporated salt is the term applied to fine crystals of salt obtained by evaporating brines, either natural or manufactured. Solar salt is applied to salt deposits obtained from shallow; pounds by sailor and Aeolian evaporation [9].
While many methods are used to purify a rock salt to analytical grade sodium chloride, there remains a gap in providing methods that will allow production of analytical grade sodium chloride at a lower cost, and allow mass production.